1. Field of the Invention
This invention relates generally to implantable prostheses for replacing human skeletal joints, and relates more particularly to implantable prostheses having friction-locking taper connections between components.
2. Background Information
Occasionally, one of the articulating skeletal joints of the human body fails to function normally as a result of disease, trauma, or congenital defect. Often, the failure is related to abnormalities in the articulating surfaces at the ends of the bones comprising the joint. In many such cases, the joint can be repaired or reconstructed by replacing the involved ends and articulating surfaces with manufactured implantable prostheses. The hip joint is among the joints most often treated with such prostheses.
The human hip joint involves two bones: the femur and the pelvis, each having a smooth articulation surface arranged for articulation against an adjacent articulation surface of the other bone. The femur includes at its proximal extremity a head having a convex, generally spherically contoured articulation surface. The pelvis, in pertinent part, includes an acetabulum having a concave, generally spherically contoured articulation surface. The mutually engaging articulation surfaces of the femur and the pelvis together form, functionally, a ball-and-socket joint.
One or both of the articulation surfaces of the hip joint may fail to perform normally, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface provided by an implantable orthopedic prosthesis. To fit defects of varying scope, while allowing healthy portions of the hip joint to be conserved, a range of types of orthopedic implants is available. The range extends from total hip prosthesis systems for replacing the articulation surfaces of both the femur and the pelvis, to simpler systems for replacing only the femoral articulation surface.
Commonly employed orthopedic hip prostheses include components that fall within one of three principle categories: femoral stems, femoral heads and acetabular cups. A so-called "total" hip prosthesis includes components from each of these categories. The femoral stem replaces the proximal end of the femur and includes a distal stem received within the medullary canal at the proximal end of the femur. The femoral head replaces the natural head and articulating surface of the femur. The acetabular cup replaces the natural socket and articulating surface of the acetabulum of the pelvis. In some designs, the stem and head are an integral, unitary component, but more often the stem and head are separate modular components designed to be assembled to suit the anatomical needs of the patient. Typically, the stem includes an integral neck portion that is assembled to the head component through a friction-locked taper connection, also known as a Morse taper. Several stems and heads, of differing sizes or configurations but having a commonly configured taper connector, can be provided to allow many different combinations of stem and head. Usually, the neck of the stem is provided with a male conical taper-locking surface and the head is provided with a complementary female conical taper-locking surface.
Head components can be varied both with regard to diameter, and with regard to the location of the center of the head relative to the stem component. The former variation is self-explanatory, whereas the latter variation is accomplished by varying the depth of placement of the female conical taper within the head. The shallower the depth of placement of the female conical taper, the greater the displacement of the center of the head from the stem when the head and stem components are assembled. A shallower placement of the female conical taper of the head results in an effective lengthening of the neck portion of the stem. For a given stem, a chosen depth of female taper in the head is designated as having a plus zero (+0) effect on neck length. Heads having shallower depths of female tapers, when mated with the particular stem, can provide extensions of the basic neck length, and are designated by the amount of neck extension they provide, e.g., plus ten millimeters (+10 mm). To assure sufficient mating surface between the male and female taper-locking surfaces, the head can be provided with an integral collar that extends from the head and surrounds the female conical taper.
Extending the effective length of the neck of a hip stem, by using a head having a shallowly placed female conical taper, increases the distance at which loads are applied to the stem through the head. This increased distance, or lever arm, with some combinations of stems and heads, can result in moment forces that exceed the design parameters of the stem. It would be desirable to provide a way to prevent assembly in the field of undesirable combinations of stems and heads. The present invention achieves this and other desirable goals.